Ugin's Nexus + Aetheric Amplifier + Karn, the Great Creator + Extruder

Ugin's Nexus

Aetheric Amplifier

Karn, the Great Creator

Extruder

How it works

1. If Karn has exactly one loyalty counter on it, activate its first loyalty ability by putting a loyalty counter on it.
2. Activate Extruder by sacrificing Ugin's Nexus. Ugin's Nexus will be exiled instead of being put into your graveyard due to its replacement effect, causing you to take an extra turn after this one.
3. Move to your next turn.
4. Activate Aetheric Amplifier by paying 4 and tapping it, doubling the number of counters on Karn.
5. Activate Karn's second loyalty ability by removing two loyalty counters from it, returning Ugin's Nexus from exile to your hand.
6. Cast Ugin's Nexus by paying 5.
7. Repeat from step 2.

This brilliant loop locks down extra turns by perpetually feeding Ugin's Nexus to Extruder, turning its destruction-replacement effect into an endless chain of bonus phases. To sustain the engine, Aetheric Amplifier steps in to double Karn, the Great Creator's loyalty counters, giving the planeswalker enough fuel to continuously fish Ugin's Nexus right back out of exile with its minus ability.